Nitrification is an important part of activated sludge wastewater treatment processes in view of the severe effluent discharge standards for nitrogen (N) that are nowadays imposed by legislation. Several on-line analyzers have been developed to achieve better monitoring and control of biological N removal processes. In this paper, the results of on-line experiments on a pilot-scale activated sludge plant with a recently developed titrimetric sensor are described. Based on a titration of an activated sludge sample, it was possible to estimate the ammonium N (NH4 +-N) concentration in the activated sludge, without the need for any sample pretreatment. The NH4 +-N concentrations obtained with the titration method were consistent with the NH4 +-N concentrations measured with an on-line analyzer, which was operated on the effluent of the pilot plant. Contrary to chemical on-line NH4 +-N analyzers, the titration method provides data about the nitrification rate of the activated sludge.

The control of nitrogen removal processes in activated sludge systems requires not only on-line values of the parameters such as ammonium, nitrate and nitrite, but also an evaluation on the nitrifying and denitrifying biomass activity, influent toxicity and carbon source requirements for denitrification. This paper describes four different biosensors, two on denitrification and two on nitrification. They all measure the biological activity of the nitrifying or denitrifying bacteria. Three of them measure nitrogen and carbon species, the fourth biosensor measures influent toxicity. These instruments open new perspectives for efficient on-line monitoring of wastewater treatment plants.